The present invention relates to a method and apparatus for detecting surface deviations from a reference plane. The invention is particularly useful in applications involving detecting such deviaions while examining a mask pattern on a semiconductor substrate, and is therefore described below with respect to such an application, but it will be appreciated that the invention could advantageously be used in many other applications as well.
Scanning electron microscopes are commonly used for the accurate indication of critical dimensions of patterns on silicone wafers for the production of integrated circuits. These indications are done when the patterns are at the focal plane of the microscope objective. Any deviation of the pattern from the focal plane affects the indications and therefore must be detected so that the lens or the pattern position can be properly readjusted.
A number of optical systems are known for determining such deviations of the pattern from the focal plane of the microscope objective. Examples of known techniques are described in an article published by H. D. Wolpert, Photonics Spectra June 1987, pp 165-168 and in U.S. Pat. No. 3,264,935. Such systems are generally based on focussing a beam of radiation (e.g., a laser beam) on the examined surface and reflecting the beam to a position-sensitive detector such that any deviation in the examined surface from the focal plane of the microscope objective produces a non-symmetry in the light spot projected on the detector with respect to a reference point on the detector.
However, such a method is sensitive to differences in reflection of the substrate. This greatly limits its use when employed for imaging a masked pattern on a semiconductor substrate for the manufacture of integrated circuits. However, patterned surfaces produce significant and non-uniform scattering, and defraction, and non-uniform reflection. This results in an error, known as a drawback, in the position indication. The drawback is not constant and depends on the random geometrical and optical structure of the pattern on the examined reflecting surface.
U.S. Pat. No. 4,356,392 discloses one system for reducing this drawback. According to the system described in that patent, an auxiliary beam is focussed along a first path to a spot on the examined surface, and is reflected back along a second path to a second spot at the location of the first spot on the examined surface. The second path includes a retro-reflector which inverts the beam directed back along the second path, such that after the double reflection to the examined surface, the two halves of the beam tend to cancel out local differences in reflection.
However, such a system suffers from the following disadvantages: (1) Since the beam used for detecting the deviation in the examined surface from a reference plane is reflected from the examined surface twice, the scattering and/or the absorption at the examined surface substantially attenuates the beam intensity, and thereby decreases the collection efficiency; the system therefore requires the use of a position sensitive detector having a very wide dynamical range, which is not attainable in some situations with the present technology.
(2) In addition, the optical beam undergoing the second reflection is not completely symmetrical with the optical beam undergoing the first reflection because of significant distortions at the time of and following the first reflection; and this non-symmetry between the beams undergoing the two reflections can result in significant positional errors.
It would therefore be highly desirable to provide a method and apparatus for detecting the deviation of an examining surface from a reference plane, in which the above-described drawbacks are reduced or eliminated. It would be particularly desirable to provide such a method and apparatus which could be used in examining patterned surfaces of semiconductor substrates for the manufacture of integrated circuits.